IMS Registration (IMS Registration for an Unauthenticated User)
Visited Network
InternetUser
Visited CN
Visited IMSDNS Server
Equipment
Subscriber
SGSN
GGSN
P-CSCFDNS Server

Home Network
Home IMS
I-CSCF

S-CSCF

EventStudio System Designer 4.0

Home CN

24-Nov-07 18:36 (Page 1)

HSS

This sequence diagram describes the IMS Registration of a terminal. The IMS registration goes through the following sequence:(1) GPRS Attach: The terminal registers to the GPRS Network.
(2) PDP Context Activation: An IP address is assigned to the terminal.
(3) Unauthenticated IMS Registration Attempt: The terminal attempts an IMS registration but is challenged by the IMS network to authenticate itself.
(4) IPSec Security Association Establishment: The terminal establishes a protected session with the IMS network.
(5) Authenticated IMS Registration: Registration is reattempted. This time the terminal is successfully authenticated and accepted.
This sequence diagram was generated with EventStudio System Designer 4.0 (http://www.EventHelix.com/EventStudio). Copyright © 2007 EventHelix.com Inc. All Rights Reserved.
GPRS Attach
GMM Attach Request

The terminal powers up and attaches to the GPRSnetwork.

GMM Attach Accept
GMM Attach Complete
PDP Context Activation
Activate PDP Context

Once the attach is completed, the terminal initiates aPDP context activation.

Create PDP Context Request
Create PDP Context Request
Activate PDP Context Accept
P-CSCF IP Address

The terminal receives an IP address for the PDP
context. The terminal also receives the IP address ofthe P-CSCF. The P-CSCF serves as the initial SIP
proxy into the IP Multimedia System (IMS).

Unauthenticated IMS Registration AttemptExtract user public identity
from ISIM

The P-CSCF IP address obtained from the PDPContext Accept message is stored.
The subscriber extracts the user public identify fromthe ISIM module of the USIM.

allocate
Subscriber side client andserver ports

The SIP terminal allocates the subscriber side clientand server ports. These ports will be included in the
REGISTER message sent to the P-CSCF.

Store P-CSCF IP Address

IMS Registration (IMS Registration for an Unauthenticated User)
Visited Network
Internet
User
Visited CN
Visited IMSDNS Server
Equipment
Subscriber
SGSN
GGSN
P-CSCFDNS Server
REGISTER

Home Network
Home IMS
I-CSCF

S-CSCFHome CN
HSS

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The subscriber sends a Register message to informthe network that the specified user public identify(myname@mynetwork.com) is available at the IP
address indicated in the Contact Header. The User
Equipment (UE) also adds a via header to record thatthe message had traversed the UE. The REGISTER
message also includes the server and client ports.
Note that the message itself is sent on the standard
SIP port 5060.
The SIP REGISTER message also includes the private
identity of the user. This identity will be used by theS-CSCF and HSS to identify the user.
The P-CSCF receives the REGISTER message anduses the DNS to translate from the domain hims.net
to the IP address of the home network.

REGISTER sip:hims.net SIP/2.0,
Via: SIP/2.0/UDP UE-IP;branch=0abab,
Route: sip:[P-CSCF-IP],
Max-Forwards: 20,
From: <sip:name@hims.net>;tag=abbb,
To: <sip:name@hims.net>,
Contact: <sip:[UE-IP]>;expires=90000,
Call-ID: ababab,
CSeq: 25 REGISTER,
Security-Client: port-s, port-c,
Authorization: Digest username = name.private@hims.net,
Content-Length: 0

DNS Querydomain = hims.net

DNS Response
ip = ICSCF-IP

P-CSCF adds a Via header and removes the Routeheader. The REGISTER message will be routed to theIP address obtained from the DNS response. Notethat the integrity protection flag is set to false tosignify that the user has not been authenticated.

REGISTERREGISTER sip:hims.net SIP/2.0,
Via: SIP/2.0/UDP
pcscf1.vims.net;branch=0aab1,
Via: SIP/2.0/UDP UE-IP;branch=0abab,
Max-Forwards: 19,
From: <sip:name@hims.net>;tag=abbb,
To: <sip:name@hims.net>,
Contact: <sip:[UE-IP]>;expires=90000,
Call-ID: ababab,
CSeq: 25 REGISTER,
Content-Length: 0,
Authorization: Digest username =name.private@hims.net integrity protection:
noUser Authorization Request

Query the HSS to assign the S-CSCF.

name.private@hims.net

User Authorization Answer

HSS replies with the S-CSCFs.

S-CSCF Name,
S-CSCF Capabilities

Select S-CSCF

REGISTERREGISTER sip:hims.net SIP/2.0,
Via: SIP/2.0/UDP
icscf1.hims.net;branch=0aab2,
Via: SIP/2.0/UDP
pcscf1.vims.net;branch=0aab1,
Via: SIP/2.0/UDP UE-IP;branch=0abab,
Route: sip:scscf1.hims.net,
Max-Forwards: 18,
From: <sip:name@hims.net>;tag=abbb,
To: <sip:name@hims.net>,
Contact: <sip:[UE-IP]>;expires=90000,
Call-ID: ababab,
CSeq: 25 REGISTER,
Content-Length: 0,
Authorization: Digest username =name.private@hims.net integrity protection:
noI-CSCF selects the S-CSCF based on the S-CSCF
capabilities.
The I-CSCF forwards the REGISTER message to theselected S-CSCF.

IMS Registration (IMS Registration for an Unauthenticated User)
Visited Network
InternetUser
Visited CN
Visited IMSDNS Server
Equipment
Subscriber
SGSN
GGSN
P-CSCFDNS Server

Home Network
Home IMS
I-CSCF

Home CN

S-CSCF
HSS
Multimedia Authentication Request

EventStudio System Designer 4.0
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name.private@hims.net

Multimedia Authentication AnswerHSS passes the Random number (RAND),
RAND, AUTN, XRES, CK, IK

Authentication token (AUT), signed result (XRES),
Cipher key (CK) and Integrity Key (IK).

Select Authentication vectors
Save the selected authentication
vector401 Unauthorized
WWW-Authenticate: nonce=RAND-AUTN, ck, ik,
Via: icscf1, pcscf1, ue-ip

401 Unauthorized
WWW-Authenticate: nonce=RAND-AUTN, ck, ik,
Via: pcscf1, ue-ip

Save CK and IK

allocate
P-CSCF side client and server ports

401 Unauthorized
WWW-Authenticate: nonce=RAND-AUTN,
Security-Server: port-s, port-c

Verify AUTN and Compute
RES

The user is currently not authenticated, so theregistration request is rejected. The terminal ischallenged to authenticate the user. RAND, AUTN, CK
and IK are passed in the WWW-Authenticate header.
Pass the message to the P-CSCF. CK and IK are
carried in the WWW-Authenticate header.
The P-CSCF saves the ciphering and integrity keys.
These keys will be needed for establishing the IPSecsecurity association.
The P-CSCF allocates the subscriber side client andserver ports. These ports will be included in the 401
Unauthorized message sent to the Subscriber.
Pass the RAND and AUTN values to the subscriber.
The CK and IK are removed from theWWW-Authenticate header. The P-CSCF side clientand server ports are also included in the message.
The message itself is sent on the standard SIP port
5060.
Authenticate the IMS network by verifying theauthentication token (AUTN). Also compute the RES
value that will be passed back to the IMS network foruser authentication.

IPSec Security Association Establishment
IPSec SA for UE Initiated Requests
UE-Client -> P-CSCF-Server

IPSec SA for Responses to UE
UE-Server <- P-CSCF-Client

IPSec SA for P-CSCF Initiated Requests
UE-Server <- P-CSCF-Client

IPSec SA for Responses to P-CSCF
UE-Client -> P-CSCF-Server

Authenticated IMS Registration

Establish IPSec security associations for all the clientand server ports.

IMS Registration (IMS Registration for an Unauthenticated User)
Visited Network
InternetUser
Visited CN
Visited IMSDNS Server
Equipment
Subscriber
SGSN
GGSN
P-CSCFDNS Server
REGISTER

Home Network
Home IMS
I-CSCF

Home CN

S-CSCFHSS

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The Subscriber has now established the IPSec
security associations with the P-CSCF. At this point,
the SIP REGISTER message is sent again. This timethe message is protected by IPSec and the message
is addressed to the P-CSCF server port passed in the401 Unauthorized message. The message contains
the RES in the Authorization header.
Pass the REGISTER message to the I-CSCF. This time
the Authorization header indicates that integrity
protection is enabled.

Via: UE-IP;UE-Server-Port,
Route: pcscf1, pcscf-server-port,
Contact: UE-IP ue-server-port,
Authorization: Digest username = name.private@hims.net
response=RES

REGISTER
Via: pcscf1 UE-IP;UE-Server-Port,
Contact: UE-IP ue-server-port,
Authorization: Digest username =name.private@hims.net response=RES integrity
protection: yes,RES

User Authorization Request

Query the HSS to assign the S-CSCF.

name.private@hims.net

User Authorization Answer

HSS replies with the S-CSCFs.

S-CSCF Name,
S-CSCF Capabilities

REGISTER
Via: icscf1 pcscf1 UE-IP;UE-Server-Port,
Contact: UE-IP ue-server-port,
Authorization: Digest username =name.private@hims.net response=RES integrity
protection: yes,RES

The SIP REGISTER message is finally delivered to theS-CSCF.

Server Assignment Request Request subscriber related information from the HSS.
name.private@hims.net

Server Assignment Answer Received subscriber related information.
Compare RES and XRES

200 OK
Via: icscf1, pcscf1, UE-IP;UE-Server-Port

200 OK

Compare the RES reported by the subscriber withe
XRES value.
The RES and the XRES matched, so the S-CSCF
replies with success.
The success is relayed back to the P-CSCF.

Via: pcscf1, UE-IP;UE-Server-Port

200 OK
Via: UE-IP;UE-Server-PortThe IMS registration of the user is now complete.